The rates of phagocytosis and pinocytosis in Acanthamoeba under several experimental conditions were affected quantitatively to the same degree, suggesting that cellular mechanisms are the same for both events. Increasing phagocytic uptake decreases the amount of pinocytic uptake, but the total volume of uptake remains constant. Pinocytosis is not "saturable" but phagocytosis ceases when about 15% of the cell volume is occupied by indigestable particles. The data suggest that the volume of an internal compartment limits the rate of endocytosis. We examined phagosome membranes with freeze-fracture replication and found an increased density of intramembranous particles in the phagosome membrane as compared to the surface membrane from which it derived. The increase in particle density occurs after separation of the phagosome from the cell surface. We are attempting to confirm and extend the morphological observations by isolating a pure phagosome membrane fraction in order to compare the protein/lipid ratio with that of the surface membrane. Efforts this year have concentrated on establishing the purity of the isolated phagosome membranes. In the coming year we will focus on elucidation of the cytological mechanism for increase in phagosome intramembraneous particle content, and the analysis of the purified membrane fractions. BIBLIOGRAPHIC REFERENCES: Ryter, A. and Bowers, B.: Localization of acid phosphatase in Acanthamoeba castellanii with light and electron microscopy during growth and after phagocytosis. J. Ultrastruc. Res. 57: 309-321, 1976. Jones. J. B., Bowers, B. and Stadtman, T. C.: Methanococcus vannielii: Ultrastructure and sensitivity to detergents and antibiotics. J. Bacteriol. 130: 1357-1363, 1977.